This invention relates to self-aligned aperture masks used in rear projection screens and, in particular, to self-aligned masks which achieve high definition of the individual apertures making up the mask.
Rear projection televisions are widely used as consumer products and are becoming ever more popular as computer monitors. A critical component of such televisions is the rear projection screen upon which the user views the ultimate image. Such screens need to satisfy a number of stringent criteria.
For example, to provide a bright image, the screen should control the distribution of light in viewer space so that as much light as possible is directed to the places where the user is likely to be. Arrays of either cylindrical lenses or microlenses can be used for this purpose. Co-pending U.S. patent application Ser. No. 60/222,033, filed Jul. 31, 2000, and entitled xe2x80x9cStructured Screens for Controlled Spreading of Light,xe2x80x9d the contents of which are incorporated herein by reference, discloses particularly preferred microlens arrays for this purpose. This application is referred to hereinafter as the xe2x80x9c""033 patent applicationxe2x80x9d.
To achieve high contrast, rear projection screens have typically included an aperture mask designed to prevent ambient light from entering the projection television. Such light can reflect from structures internal to the television and become redirected to the viewer. This redirected light reduces the contrast of the image since it can appear at, for example, places where the image should be black.
In their most simple form, aperture masks can be prepared by printing a black matrix on one of the surfaces making up the rear projection screen. Beginning with arrays of cylindrical lenses and continuing through to arrays of microlenses, workers in the art have used self-alignment techniques to form such aperture masks. See, for example, U.S. Pat. No. 2,338,654, U.S. Pat. No. 2,618,198, U.S. Pat. No. 5,870,224, PCT Patent Publication No. WO 99/36830, EPO Patent Publication No. 1 014 169 A1, Japanese Patent Publication No. 2000-147662, and Japanese Patent Publication No. 2000-147663.
The goal of these self-alignment techniques is to ensure that the apertures of the mask correspond to the locations where light will be focused by the lens array. In some cases, the apertures are produced first and are subsequently used to produce microlenses (see U.S. Pat. No. 5,897,980, EPO Patent Publication No. 0 753 791, PCT Patent Publication No. WO 99/36830) and in other cases, the lenses are prepared first and then used in the production of the apertures (see U.S. Pat. No. 4,666,248).
The present invention is concerned with the second approach, i.e., the approach in which an array of lenses is produced first and then used to create a self-aligned aperture mask.
The earliest self-aligned aperture masks were based on photographic emulsion technology. This approach has a number of disadvantages including low transparency of the apertures and poor age performance. If a photographic emulsion were to be used to produce an aperture mask for a projection television, the age performance problem would only become worse in view of the heat which such televisions generate.
Recently, proposals have been made to use DuPont""s Cromalin(copyright) system to produce aperture masks. See, for example, Japanese Patent Publication No. 09-269546 and U.S. Pat. No. 5,870,224.
As discussed in these references, in the first step of the process, a laminate of (1) a tacky, photosensitive, transparent layer and (2) a lenticular lens array is prepared. Thereafter, the photosensitive layer is exposed with UV light through the lens array. The UV light causes the photosensitive layer to polymerize and lose its tackiness in the exposed regions. A backing film carrying toner particles is then applied to the photosensitive layer and through the application of pressure (e.g., pressure up to 700 kg/cm2), toner particles are transferred to the unexposed and thus still tacky regions of the photosensitive layer. The backing film is then mechanically pulled away from the photosensitive layer with toner particles remaining on the layer at the tacky regions.
As illustrated by the comparative example set forth below, the Cromalin(copyright) system has the serious drawback that it produces apertures of low definition, i.e., rather than having cross-sectional perimeters of the desired design shape, the apertures have ragged, uneven perimeters. Moreover, the Cromalin(copyright) system for producing aperture masks exhibits a high level of variability in the shapes of the apertures, both among the apertures of a given screen and between screens.
An additional problem with the Cromalin(copyright) system arises from the fact that the apertures are filled with polymerized material. Such material may vary from aperture to aperture in, for example, the degree of polymerization, which can produce uncontrollable variations in the optical path for light passing through the finished screen. For example, the surface curvature of the polymerized material filling the apertures can vary in an uncontrollable manner from aperture to aperture.
Further variability in masks produced using the Cromalin(copyright) system can result from different amounts of toner being transferred to the regions of the mask which are intended to block visible light.
A further problem with the Cromalin(copyright) system arises from the fact that the high pressures used in the toner transfer step can damage the lens array portion of the laminate.
The present invention is designed to overcome these problems with the Cromalin(copyright) system.
In view of the foregoing, there is a need in the art for a self-aligned aperture mask having some and preferably all of the following properties:
(1) the mask comprises apertures of high definition;
(2) the mask is produced by a process with low variability both for the apertures of an individual mask and between masks in terms of aperture configuration and optical properties;
(3) the mask is produced by a process which produces essentially the same level of blackness at all parts of the mask which are intended to block light;
(4) the mask is produced by a process which does not apply excessive pressure to a lens array used to produce the self-aligned apertures;
(5) the mask does not substantially deteriorate with age; and
(6) the mask can be produced economically on a continuous basis.
The present invention provides self-aligned aperture masks which have some and preferably all of the foregoing six properties.
In accordance with one of its aspects, the invention provides a method for making an aperture mask for a screen comprising:
(a) providing a substrate having a first side and a second side, the first side having an array of lenses associated therewith;
(b) applying a positive-acting photoresist to the second side of the substrate, said photoresist comprising at least one pigment dispersed in a photosensitive matrix which, upon exposure to actinic radiation, becomes soluble in a developer solution, said at least one pigment absorbing light in the visible range;
(c) exposing the positive-acting photoresist by passing actinic radiation through the array of lenses to form an exposure pattern in the photoresist, said exposure pattern being a function of the optical properties of the lenses; and
(d) developing the positive-acting photoresist using the developer solution to remove photoresist which has been exposed to the actinic radiation and thus form apertures in the photoresist which will transmit visible light to a greater extent than regions of the photoresist that have not been exposed to the actinic radiation.
In accordance with another of its aspects, the invention provides apparatus (a screen subassembly) comprising:
(a) a substrate having a first side and a second side;
(b) an array of lenses associated with the first side of the substrate; and
(c) an aperture mask on the second side of the substrate comprising:
(i) a pigmented polymer layer comprising at least one pigment dispersed substantially uniformly throughout the layer, said pigment absorbing visible light, said pigmented polymer layer being either an unexposed positive-acting photoresist or an exposed, but not developed, positive-acting photoresist; and
(ii) a plurality of apertures which pass through the pigmented polymer layer, the locations of the apertures being based on the optical properties of the array of lenses; wherein the apertures transmit visible light to a greater extent than the pigmented polymer layer.
As used herein, the term positive-acting photoresist means a photoresist having the characteristic that development of the photoresist causes removal from the photoresist of those portions of the photoresist that have been exposed to actinic radiation.
Additional features and advantages of the invention are set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.
The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. The drawings are not intended to indicate scale or relative proportions of the elements shown therein.